1. Field of the Invention
This invention pertains to floating drilling vessels and, more particularly, to heave compensators for supporting the drill string on such vessels.
2. Description of the Prior Art
Heave compensators for maintaining substantially constant pressure on the drilling bit of drill strings are well known. Examples of drilling derricks employing two types of heave compensators are illustrated in U.S. Pat. Nos. 3,158,206 and 2,945,677. In both of these patents the compensator, which includes a cylinder and piston, is supported in such a manner as to require the permanent extension of the derrick height by an amount at least equal to the stroke of the piston in the compensator. The stroke is normally designed for movement of 25 feet or more, thus adding considerable height to the derrick. In the former patent the cylinder of the compensator is secured to the top of the derrick, thus forming the permanent extended height. In the latter patent the cylinder is interposed between the traveling block and the rotatable drill string supporting means, such that the top of the derrick must be extended to accommodate the cylinder length.
Another difficulty with some prior heave compensators is that the drawworks are incorporated in the compensation system requiring constant cycling of the supporting cables due to wave action.